Element z has two natural isotopes: z-79 (78.918 amu) and z-81 (80.916 amu). calculate the atomic mass of element z given the abundance of z-81 is 49.31%.

Isotopes are atoms of the same element with different number of neutrons, hence different mass numbers.
relative atomic mass is the weighted average atomic masses of the isotopes.
atomic mass of element z =78.918 amu x (100-49.31)%+ 80.916 amu x 49.31%
 atomic mass = (78.918 x 50.69%) + (80.916 x 49.31%)
                      = 40.00 + 39.90 
 atomic mass of element z = 79.90 

Ijust took the quiz. their is no coefficient needed. : )

True or false:   the purpose of a defense mechanism is to get uncomfortable feelings out into the open so they can be dealt with in a healthy manner.indeed true: )